1. Field of the Invention
The present invention relates to an ink cartridge for use in an ink jet printer, and, more particularly, to an ink cartridge having a porous foam therein.
2. Description of the Related Art
An ink cartridge for use in an ink jet printer typically includes a porous member disposed therein. The porous member is disposed in fluid communication with a printhead assembly, and supplies ink to the printhead assembly during periods of operation. Typically, the porous member is in the form of a felted piece of polyurethane foam. The foam is thermally compressed, or felted, until it holds a compression set at roughly a 3 to 1 compression ratio. That is, the foam is heated close to its melting point under a compression loading and allowed to thereafter cool, thereby resulting in a denser foam with an increased effective porosity. When felted, the foam achieves an effective porosity of approximately between 150 and 200 pores per inch (ppi).
Conventional ink cartridges typically use a water soluble, dye based ink which is applied to a print medium. The dots of dye based ink combine to form characters and graphics on the print. medium which is subject to print quality variations, including fuzziness, bleed, ink spread, and lack of optical density.
The print quality of the ink jet printer can be greatly improved through the use of a higher viscosity, higher surface tension ink, such as a pigmented ink. The pigments, e.g., carbon black pigments or color pigments, are suspended in the water as opposed to being dissolved in the water. After the ink is applied to the paper and the water dries, the non-water soluble pigments are essentially unaffected by subsequent contact with water. Moreover, the increase in viscosity and surface tension allows the ink to set on top of the print medium with reluctance to spread into other (color) inks, thus preventing bleed of different inks. Further, the higher surface tension ink also inhibits the penetration of the ink into the paper, improving optical density. Finally, the more viscous ink does not tend to travel along the fibers of the paper, creating a crisper, cleaner print without fuzziness.
A problem with using a higher viscosity, pigmented ink is that "starvation" of the printhead assembly occurs when the higher viscosity ink is placed in a conventional, felted foam. More particularly, as the ink is jetted from the printhead assembly during operation in known manner, an adequate supply of the higher viscosity ink must flow from the felted foam to the printhead assembly so that the ink may be continuously jetted therefrom. However, the felted foam has an effective porosity of between approximately 150 to 200 ppi which does not allow an adequate flow rate of ink from the foam to the printhead assembly. The capillary force within the felted foam having such an effective porosity holds the higher viscosity ink therein and does not provide a sufficient flow of ink to the printhead assembly. This results in "starvation" of the printhead assembly because of the inadequate flow of ink.
One known solution allowing the use of a higher viscosity ink in an ink cartridge is to utilize a bladder-spring mechanism within the cartridge. The bladder maintains an appropriate back pressure on the ink within the ink cartridge and avoids the problem of starvation such as occurs when a higher viscosity ink is used with a felted foam.
The present inventors have also attempted to utilize a higher viscosity ink with a felted foam having a lower effective porosity, but have encountered problems associated therewith. To wit, a felted foam having an effective porosity of between approximately 60 to 80 ppi was used in conjunction with a higher viscosity, pigmented ink. It has been found that to prevent starvation of the printhead assembly during operation, the felted foam must have an effective porosity which does not exceed a range of between approximately 80 and 90 ppi. However, it has also been found that with a felted foam having a porosity of less than about 90 ppi, the printhead assembly may drip ink or "drool", during periods of inoperation. Such drooling is obviously not desirable. Thus, in order to prevent the problem of starvation which may occur during dynamic fluid conditions, the drooling problem results during static fluid conditions.
It is believed by the present inventors that the overlapping problems of starvation and drooling which occur when attempting to use a felted foam in conjunction with a higher viscosity ink are a result of the felting process. When the foam is felted, as described above, the temperature of the foam is increased near the melting point and a compression loading is temporarily applied to the foam. Because of temperature and loading gradients which exist on the foam during the felting process, the pores at the periphery of the foam may be smaller than the pores in the center of the foam. The smaller pores at the periphery of the foam may not allow, adequate fluid flow therethrough and thus cause the starvation problem during periods of operation, while the larger pores in the center of the foam may not provide sufficient capillary force to prevent drooling of the printhead assembly during periods of inoperation.
Using a felted foam also has other associated problems. For example, during the felting process chemical residue is created and deposited on the surface of the foam. This residue can react with the ink to modify the ink and create defects in print quality, or can be carried by the ink to the printhead assembly where it may clog the nozzles and possibly result in printhead assembly failure.
What is needed in the art is an ink cartridge for use in an ink jet printer which economically and reliably allows for the use of a higher viscosity, pigmented ink.